Secondary aluminum melting plants provide the processing and melting of aluminum scrap, metallurgic treatment and the refinement of melts as well as the casting of blocks or the further transportation in liquid form.
Scrap material is melted, for example, clean or painted sheet scrap, foils, new or old foundry scrap, dross and shavings.
A conventional secondary aluminum melting plant operates according to the following process. The scrap is sorted and processed. Scrap which is uniform with respect to its proportion of foreign material and alloys is directly melted. Shavings are thermally purified and dried in processing equipment for shavings. Aluminum pellets are mechanically processed in grinding and sifting devices.
The melting of processed scrap takes place batch-wise in rotary drum kilns with a melting salt, which takes up contaminations and which is renewed when necessary. The melting salt from natural sources consists mainly of a mixture of common salt (NaCl, KCl) and the addition of about 2% CaF.sub.2. About 300 to 400 kg salt is required to remelt one ton of metal alloys. The used melting salt is poured off into open containers as a salt slag and intermediately stored after hardening to a compact mass until transportation to the salt slag recovery plant. The metal melt from the rotary drum furnace is transferred to heat retention furnaces (converters) in which the melt is metallurgically treated, post-purified (refined) and made into alloys. The refinement takes place by introducing refining gases (chlorine, nitrogen, argon). The materials separated in this treatment float to the top and are separated as dross. This dross is then processed and remelted. The processed melt is then cast as blocks or idled into insulated containers for liquid transport to a foundry.
The waste gases of the drum furnace and the heat retention furnace (converter) are fed to a waste gas channel system. The raw gases are fed via sheet metal piping to a heat exchanger. The final separation of particulates takes place in subsequent flat tube filters. The particulate matter is a central particulate disposal point. The purified waste gas flows through a subsequent draft fan and chimney to the atmosphere. The purified gas values correspond to the German emissions specification TA Luft 1986.
Today, however, more stringent requirements are made on waste gas purification which can no longer be realized in conventional smelting plants. Problems arise in particular with respect to the maintenance of the total carbon value and the limits for dioxin, to be issued in the near future.
The object of the invention therefore is to provide a process and an apparatus with which the waste gases arising when melting and treating aluminum scrap in a secondary melting plant can be effectively purified and freed of hazardous substances.